Place chicken breasts in a stock pot with half an onion and salt. Cover with water, bring to a boil, and simmer for about 20 minutes until cooked through. Remove and let cool slightly. Shred with two forks.
Mince the remaining half of the onion and mix it into the shredded chicken.
Wrap 6 tortillas in a damp paper towel and microwave for 40-50 seconds until pliable. Dip each in green salsa, then scoop about 2 tablespoons of filling into each tortilla, roll up, and place seam side down in a baking dish.
Drench the assembled enchiladas with 1 cup of salsa verde and sprinkle with crumbled queso fresco. Repeat with remaining tortillas.
Cover the dish with foil and bake in the preheated oven for 25-30 minutes until hot and bubbly.
Serve warm topped with sour cream and chopped cilantro if desired.
Notes
Feel free to customize with additional toppings like avocado or jalapeños.